1. Field of the Invention
This invention relates to a switching power supply unit, and more particularly to a switching power supply unit using RCC (Ringing Choke Converter) and a switching power supply unit such as a PWM control type fly-back converter or a forward converter.
2. Description of the Related Art
Firstly, a patent document 1 (Japanese Patent Application Laid-Open No. 5-236743) is described below as a conventional example 1. This conventional example 1 relates to a switching power supply unit which stabilizes voltage control even against a wide range of load variation.
FIG. 6 shows an exemplary circuit of the switching power supply unit. This switching power supply unit employs the RCC as its basic structure, and is provided with a light load stabilization circuit 50. The light load stabilization circuit 50 automatically detects whether or not a load 100 is large (or small) to separate (or connect) a dummy load 40.
In the secondary DC power supply 30 side, there is provided the light load stabilization circuit 50, which comprises a switch circuit (transistor) 51 connected in series to the dummy load 40 and a voltage detection circuit 52. The voltage detection circuit 52 comprises a zener diode ZD and voltage dividing resistors R1 and R2, and turns on the transistor (51) when the detected voltage exceeds a set voltage, and otherwise turns off the transistor (51).
In this structure, the dummy load 40 is separated from the secondary DC power supply 30 when the load 100 is large. Accordingly, firstly a large value of the dummy load 40 can be selected without taking into account the increase of power consumption and the reduction of power efficiency at the time of a maximum load. Secondly, a small value of the dummy load 40 can also be selected in a range in which the control can be stabilized even against a minimum-side variation of the load 100. Specifically, the size of a pulse transformer 20 can be reduced by maximizing the oscillation frequency. Thus, the value of the dummy load 40 can appropriately be selected in view of the conditions described above.
In this switching power supply unit using RCC, when the load 100 is large, the light load stabilization circuit 50 separates the dummy load 40 from the secondary DC power supply 30. On the other hand, when the load 100 becomes small (a state of light load), the oscillation frequency becomes high, so that the control becomes unstable and secondary DC power supply voltage V2 becomes high. Therefore, when the detected voltage exceeds the set value, the voltage detection circuit 52 turns on the transistor (51) and thereby connects the dummy load 40 to the secondary DC power supply 30. Consequently, the apparent load (100) increases, so that the oscillation frequency is reduced and the secondary DC power supply voltage V2 can be stabilized.
As described above, there is provided the light load stabilization circuit 50 which comprises the switch circuit 51 and the voltage detection circuit 52. And, when the load 100 becomes small, the oscillation frequency becomes high, and it becomes difficult to stabilize the control, the dummy load 40 is connected, otherwise, the dummy load 40 is separated. Accordingly, even when the load 100 has a wide range of variation (from the maximum to the minimum), it is possible to stabilize secondary DC power supply voltage V2, save power, and maintain a predetermined power efficiency.
Also, the voltage detection circuit 52 comprises a zener diode ZD and voltage dividing resistors R1 and R2. Therefore, it is easy to select of the set voltage etc., coverage of the unit is wide, and it is easy to handle the unit.
Next, a patent document 2 (Japanese Utility Model Application Laid-Open No. 5-80185) is described below as a conventional example 2. This conventional example 2 relates to a power supply circuit which can achieve a stable power supply even against an input voltage having a wide range. FIG. 7 is a view showing an example of the power supply circuit.
As shown in FIG. 7, a control circuit 8 varies the period of a voltage applied to the base of a switching transistor 9 according to a variation of a load connected between output terminals A and B positioned in the secondary side of a power supply transformer 7. By this action, an amount of current per unit time flowing through the primary side of the transformer is controlled to keep constant the voltage between A and B.
A variation of the load between the output terminals A and B is detected by voltage dividing resistors 10 and 11, a shunt regulator 12, a resistor 13 and a photodiode 14, all provided in the secondary side, and a photo transistor 15 provided in the primary side. Specifically, a variation of voltage between A and B corresponding to the above load variation is detected by the shunt regulator 12. And then, the amount of variation is transmitted to the primary side via the photodiode 14 and the photo transistor 15. By this action, the amount of current flowing through the primary coil is controlled so as to eliminate the variation of voltage between the above terminals, and then the voltage between A and B is kept at a constant value at all times.
Referring to FIG. 7, reference numerals 16 and 17 denote a diode and a smoothing capacitor, respectively, for converting a squire-wave voltage induced in the secondary coil into a direct current. Reference numerals 18 and 19 denote a rectifier and a smoothing capacitor, respectively, for converting an alternating voltage applied from an alternating power source 20 via a filter 21 into a direct current. Reference numeral 22 denotes a snubber circuit for suppressing squire-wave current noises applied to the primary coil.
By this structure, a DC voltage proportional to an input voltage is produced by a rectifier diode 5 and a rectifier capacitor 6, and current flowing through a dummy resistor 1 is controlled by a transistor 2. The amount of current flowing through the dummy resistor 1 becomes larger when the voltage applied to the base of the transistor 2 becomes lower, or when the input voltage becomes higher. This is equal with the addition of a larger load. A resistor 3 and a constant-voltage diode 4 are used to adjust the current flowing through the dummy resistor 1.
As described above, according to the conventional example 2, when the input voltage is high, a large dummy load is added, and when the input voltage is low, a small dummy load is added. Consequently, a dummy load corresponding to the input voltage is added to prevent intermittent oscillation, and further a load is not uselessly added.
In the conventional example 1, however, when the output voltage becomes high and exceeds a predetermined set voltage (a voltage detected by a zener diode), a current is flowed through the dummy load. Accordingly, variation of the output voltage due to the load variation is considerably large. Also, since the output voltage is detected by the zener diode, variation of the detecting point is large. Also, since there is no hysteresis in at a time of switching of the dummy load, chattering often occurs in the operation.
Further, in a switching power supply unit using RCC, according to the basic operation of RCC, when the load becomes light, the oscillation frequency becomes high and at the same time the on-period becomes short, so that it becomes impossible for the main switching element to secure the on-period. Accordingly, intermittent oscillation occurs. As a result, the output ripple voltage often becomes significantly large or the output voltage often rises.
In the conventional example 2, the dummy load is connected at all times, and the amount of load is controlled according to the magnitude of the input voltage. Accordingly, power loss is large, and a large size transistor is required.